The present invention relates to an image signal-generating circuit which horizontally compresses dots which form an image pattern.
Projection TV receivers have larger viewing screens that TV receivers which are directly viewed. The clearness of the image formed on such a projection TV receiver is not sharp because red, green and blue scanning lines are not in focus. Therefore, when the direction of the projection tube is changed or its altitude is adjusted, it is necessary to achieve convergence using an adjusting image pattern, such as a crosshatch pattern or dot pattern. Where an image signal-generating circuit for generating an image pattern used for achieving convergence is fabricated entirely of hardware, various circuits must be provided for different image patterns. This makes the configuration of the circuit complex. If a cursor is to be displayed on the viewing screen, the circuit is more complex. Consequently, this approach is impractical.
Accordingly, an improved method has been introduced. This method is now described by referring to FIG. 3, where an image signal-generating circuit is generally indicated by reference numeral 1. The circuit 1 includes a memory 2 in which information about one field or one frame of image is stored. The image pattern read from the memory 2 is supplied to a video-processing circuit 4 included in a projection TV receiver 3. The signal-generating circuit 1 also includes a central-processing unit 5 to which the memory is connected. An address-generating circuit 6, also incorporated in the circuit 1, operates in phase with a horizontal synchronizing signal. Data is read from the memory 2 at addresses specified by the address-generating circuit 6 and fed to the video-processing circuit 4 via a converter circuit 7. Converter circuit 7 converts parallel data into serial form. The image information stored in the memory 2 can be altered at will within the capacity of the memory 2 by making use of the image-drawing function of the central-processing unit 5.
When the aforementioned image signal-generating circuit 1 displays a crosshatch pattern, for example, on the viewing screen, each horizontal line can be drawn at the width of one line, but the width of each vertical line is larger because of the restricted storage capacity of the memory 2. Therefore, restrictions are imposed on the minimum width of dots which can be realized in software due to the storage capacity of memory 2. Thus, it is difficult to make the width of the vertical line substantially equal to the width of the horizontal line. A check is performed to see if the red, green and blue scanning lines are out of focus while viewing a rectangular portion surrounded by vertical and horizontal lines which intersect at four points, in order to attain convergence. For the above reason, it is difficult to obtain an image pattern best suited for this check operation.